Ovens used in laboratories and/or as incubators must have carefully maintained temperature control. It is also important to be able to arrive at a selected temperature as quickly as possible. Obviously, the greater the power output of the heater elements used in such ovens, the greater the heat that can be delivered to the oven chamber and the faster the ultimate temperature can be reached. If the heater elements are de-energized precisely at the desired temperature, the heater elements remain hot as do the walls of the oven chamber. The temperature of the chamber continues to increase despite the heater elements having been de-energized. This phenomenon is commonly referred to as "overshoot". For example, if a particular oven has an operating range of 0.degree.-300.degree. C., and the desired temperature is 150.degree. C., de-energization of the heater elements at such temperature will result in the temperature increasing well beyond 150.degree. C. The heat from the heater elements will finally dissipate and the temperature of the chamber will drop to a temperature below 150.degree. C., whereupon the heater elements are automatically energized to deliver more heat. Ultimately, the oven temperature will reach a substantially quiescent state at 150.degree. C. It can be appreciated, however, that the time to arrive at this condition would be excessive, without some temperature control circuit.
In the past, heater control circuits have automatically maintained the heater elements fully on up to some predetermined temperature less than the desired temperature. When such predetermined temperature was reached, these circits de-energized the heater elements for a duration each cycle, which duration increased as the ultimate temperature was neared; in other words the interruptions become longer and longer. At the desired temperature, no power was supplied to the heater elements and they were theoretically cool. For example, a 150.degree. ultimate temperature may be attained by energizing the heater elements intermittently beginning at, say, 140.degree.. At the temperature the heater elements would be energized during, for example, 90% of each cycle and during the remaining 10%, they would be de-energized. As the temperature continued to rise, the period of energization during each cycle would steadily decrease, so that by the time the temperature reached say 148.degree., the heater elements were only operated 20% of each cycle. Basically, such temperature control was furnished by modulating a triangular wave onto a voltage established by the setting of the desired temperature in the chamber. Such circuits are disclosed in U.S. Pat. No. 3,842,243 to Gregory and U.S. Pat. No. 3,584,291 to Budniak.
The problem with these prior art circuits is that the amount of temperature control of proportioning, as it is called, is fixed. In the foregoing example, it was assumed that the circuit design caused intermittent operation of the heater elements to start at 140.degree. when the desired temperature was 150.degree.. That fixed the starting point at 10.degree. less than the ultimate temperature throughout the entire temperature range. At a desired temperature of 200.degree., intermittent operation of the heater elements would start at 190.degree.. At 300.degree., the intermittent operation would start at 290.degree., and so forth.
With such fixed proportioning, the heater elements started intermittent operation too high at lower temperatures, and too low at higher temperatures. As a result intermittent operation is not sufficient to prevent overshoot at lower temperatures. In such case, the oven temperature began to fall after it reached its maximum, ultimately setting at the desired temperature. However, such performance was undesirably time-consuming. In the above example, the control circuit was set at the factory so that, when the desired temperature was 150.degree., the heater elements began operating intermittently at 140.degree.. If a temperature of 100.degree. was selected, then intermittent operation would start at 90.degree.. The temperature might increase to 102.degree. or 103.degree., and then oscillate to the desired temperature.
In the same vein, for desired temperatures above the temperature for which the control circuit was set at the factory, the desired temperature was either never reached (commonly called "droop") or reached only after an excessively long time.
The same comments made above with respect to temperature in an oven chamber are also applicable to other characteristics of the gas in the chamber. For example, it may be desirable to control the level of a particular ingredient in the gas such as carbon dioxide or its relative humidity.